As a transmission system which can increase a transmission capacity of an optical communication system, Japanese Unexamined Patent Application Publication No. 2001-036505 describes a system which makes polarized waves orthogonal to each other between light pulses corresponding to adjacent bits on a time axis, such that polarization multiplexing is used together with time-division multiplexing. Specifically, a multiplexing optical circuit described in Japanese Unexamined Patent Application Publication No. 2001-036505 is configured to make polarized waves of two separately modulated light beams orthogonal to each other, to apply a time delay half the pulse interval of each modulated light beam between the two modulated light beams, and to then perform polarization combine. According to this system, polarized waves are made orthogonal to each other between adjacent bits, such that interference of light pulses does not occur. For this reason, even if the waveforms of the light pulses temporally overlap, there is little influence on the transmission characteristics. Accordingly, it is possible to increase the transmission capacity without depending on a method of narrowing the light pulses.
In the configuration of Japanese Unexamined Patent Application Publication No. 2001-036505, only with the use of an optical path difference between two polarized waves (modulated light beams) in a birefringent crystal which performs polarization combine, the above-described time delay necessary for polarization multiplexing and time-division multiplexing is generated between the two modulated light beams. On the other hand, since the time delay is determined at the bit rate of the system, it is difficult to freely select the optical path difference in the birefringent crystal, consequently, the length (the crystal thickness in the light propagation direction) of the birefringent crystal. For this reason, it is necessary to use a comparatively large type as the birefringent crystal, causing a problem in that the apparatus may not be reduced in size.